Superconductivity refers to that state of metals and alloys in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature ("T.sub.c ")
Until recently, attaining the T.sub.c of known superconducting materials required the use of liquid helium and expensive cooling equipment. However, in 1986 a superconducting material having a T.sub.c of 30K was announced. See, e.g., Bednorz and Muller, Possible High Tc Superconductivity in the Ba--La--Cu--O System, Z.Phys. B-Condensed Matter 64, 189-193 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors (HTSCs). Currently, superconducting materials having critical temperatures in excess of the boiling point of liquid nitrogen, 77K at atmospheric pressure, have been disclosed.
HTSCs have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices. More particularly, the applicant's assignee has successfully produced thin film thallium superconductors which are epitaxial to the substrate. See, e.g., Olson, et al., Preparation of Superconducting TlCaBaCu Thin Films by Chemical Deposition, Appl. Phys. Lett. 55, No. 2, 189-190 (1989), incorporated herein by reference. Techniques for fabricating and improving thin film thallium superconductors are described in the following patent and copending applications: Olson, et al., U.S. Pat. No. 5,071,830, issued Dec. 10, 1991; Controlled Thallous Oxide Evaporation for Thallium Superconductor Films and Reactor Design, U.S. Pat. No. 5,139,998, issued Aug. 18, 1992; In Situ Growth of Superconducting Films, Ser. No. 598,134, filed Oct. 16, 1990; Passivation Coating for Superconducting Thin Film Device, Ser. No. 697,660, filed May 8, 1991; and Fabrication Process for Low Loss Metallizations on Superconducting Thin Film Devices, Ser. No. 697,960, filed May 8, 1991, all incorporated herein by reference.
High temperature superconducting materials are now routinely manufactured as films having surface resistances significantly below 500 .mu..OMEGA. measured at 10 GHz and 77K. These films may be formed into resonant circuits. Such superconducting films when formed as resonators have an extremely high quality factor ("Q"). The Q of a device is a measure of its lossiness or power dissipation. In theory, a device with zero resistance (i.e. a lossless device) would have a Q of infinity. Superconducting devices manufactured and sold by applicant's assignee routinely achieve a Q in excess of 15,000. This is high in comparison to a Q of several hundred for the best known non-superconducting conductors having similar structure and operating under similar conditions.
Superconducting thin films formed as resonators have the desirable property of having very high energy storage in a relatively small physical space. Such superconducting resonators are compact and lightweight. Another benefit of superconductors is that relatively long circuits may be fabricated without introducing significant loss. For example, an inductor coil of a detector circuit made from superconducting material can include more turns than a similar coil made of non-superconducting material without experiencing a significant increase in loss as would the non-superconducting coil. Therefore, a superconducting coil has increased signal pick-up and is much more sensitive than a non-superconducting coil.
Typical resonant circuits are generally limited in their application due to their signal-to-noise ratios ("SNR"). For example, the SNR in a pickup coil of a MRI detector is a limiting factor for low-field MRI systems. Although the low-field MRI systems have a number of advantages over high-field MRI (including cost, site requirements, patient comfort and tissue contrast), they have not yet found wide-spread use in the U.S. because, in part, of their lower SNR. Resonant circuits made from superconductors improve SNR for low-field human imaging. Therefore, an appropriate superconducting resonant circuit, depending on the field level, coil type, and imaging region, will enable wide-spread use of low-field MRI.
An MRI detector including a low temperature superconducting coil and capacitor has been described. See, e.g., Rollwitz, U.S. Pat. No. 3,764,892, issued Oct. 9, 1973. In addition, resonant circuits for use as MRI detectors which include high temperature superconducting coils and non-superconducting capacitors have been described. See, e.g., Wang, et al., Radio-Frequency Losses of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Composite Superconductors, Supercond. Sci. Technol. 1, 24-26 (1988); High Tc Used in MRI, Supercond. Indus. 20 (Winter 1990); and Hall, et al., Use of High Temperature Superconductor in a Receiver Coil for Magnetic Resonance Imaging, Mag. Res. in Med. 20, 340-343 (1991). Furthermore, devices including high temperature superconducting capacitors and inductors having various structures which may be used, for example, in resonant circuits for use in MRI detectors have been described. See copending patent application: James, et al., SUPERCONDUCTING CONTROL ELEMENTS FOR RF ANTENNAS, Ser. No. 07/934,921, filed Aug. 25, 1992, incorporated herein by reference. Resonant circuits made from HTSCs enjoy increased SNR and Q values.
However, HTSC sensors must be cooled to T.sub.c temperatures and, when they are used as MRI detectors, they must be cooled without exposing a patient to danger. In addition, any apparatus in which the sensor is contained must not interfere with detection of the MRI signal. A safe, unobtrusive, and non-interfering apparatus which could be used to contain a HTSC MRI detector at its T.sub.c has not been shown.
Different approaches to this problem are possible. First, an apparatus could be made which uses an active refrigeration unit to cool the high temperature superconductor detector. However, such refrigeration units are generally bulky, noisy, and magnetic. A bulky or noisy apparatus would not be preferable for use with a patient. A device which generates magnetic fields could not be used near a HTSC MRI detector because the magnetic fields would scramble the signals the HTSC sensor was to detect. In addition, it would be difficult to build an active refrigeration unit of material which would not interfere with the detection of the MRI signal (e.g. built of material which was non-conductive and low loss dielectric).
Second, a Joule-Thomson cooler could be used. However, Joule-Thomson coolers require a large source of compressed gas held at high pressure. Maintaining a large source of highly pressurized compressed gas is expensive and requires cumbersome equipment. For example, a system using a Joule-Thomson cooler would require a long high pressure tubing leading from the source of the pressurized gas to the apparatus containing the HTSC detector.
Third, a device could be constructed which uses a coil and tubing system through which liquid nitrogen from a remote reservoir is pumped to cool a HTSC MRI detector. However, such a device would be bulky and difficult to use. In addition, materials for the coil and tubing must be flexible at liquid nitrogen temperatures and must be well insulated to avoid temperature conduction with the surroundings. Furthermore, it would be difficult to build such a device out of materials which would not interfere with detection of the MRI signal.
As mentioned above, a high temperature superconductor sensor for use as a MRI detector must be cooled to cryogenic temperatures (i.e. below its T.sub.c). In addition, any apparatus in which the sensor is contained must not interfere with detection of the MRI signal and must be safe and unobtrusive when the MRI is being performed. No optional solution has been proposed heretofore.